Chalcogenide materials are presently of great interest for use in resistance variable memory devices compared to memory technologies currently in use, due to potential advantages in switching characteristics, non-volatility, memory speed, reliability, thermal characteristics, and durability. Research in this area is reported in the articles “High Speed Memory Behavior and Reliability of an Amorphous As2S3 Film doped with Ag” by Hirose et al., Phys. Stat. Sol. (1980), pgs. K187-K190; “Polarity-dependent memory switching and behavior of Ag dendrite in Ag-photodoped amorphous As2S3 films” by Hirose et al., Journal of applied Physics, Vol. 47, No. 6 (1976), pgs. 2767-2772; and “Dual Chemical Role of Ag as an Additive in Chalcogenide Glasses” by Mitkova et al., Physical Review Letters, Vol. 83, No. 19 (1999), pgs. 3848-3851, the disclosures of which are incorporated herein by reference.
In many memory cell designs employing chalcogenide materials, a film of silver-selenide (Ag2Se) is incorporated with a chalcogenide material layer. The silver-selenide film is important for electrical performance. Accordingly, silver-selenide deposition is an important aspect of fabricating the resistance variable memory device. Most available research in silver-selenide deposition is limited and evaporation deposition is normally chosen for silver-selenide film formation.
Silver-selenide deposition by evaporation has an attendant problem because the dissociative properties of silver-selenide make it impossible to achieve precision stoichiometries of silver-selenide. It is believed that in evaporation techniques, as the silver starts to diffuse to a lower concentration, it begins to agglomerate. As the silver is tied up in clusters or agglomerates, selenium is more readily available for evaporation in the beginning of the evaporation process. Thus, during evaporation techniques, selenium is evaporated more quickly, causing the deposition target to become silver-rich. Near the end of the evaporation process little to no selenium is left for deposition onto the substrate, leaving mostly silver available for deposition. Thus substantial amounts of selenium are deposited on the substrate followed by deposition of primarily silver. Accordingly, the evaporation technique therefore does not uniformly deposit the silver-selenide and controlling the stoichiometry and surface morphology of evaporated silver-selenide is difficult.
Furthermore, evaporation deposition is not conducive to industrial application. Sputter deposition is more readily available for industrial processes and sputter deposition has many advantages compared to evaporation deposition techniques. For example, sputter deposition provides better film thickness and quality control.
Generally, sputter deposition, or sputtering, is performed by placing a substrate in a deposition chamber which is evacuated or pressurized to a desired pressure. A particle stream of the film material usually generated from a target is then generated within the chamber and the deposition occurs by condensation of the particles onto the substrate. In another sputtering technique, often referred to as ion beam bombardment sputtering, a high-energy source beam of ions is directed toward the target. The force of the bombarding ions imparts sufficient energy to the atoms of the target to cause the energized atoms to leave the target and form a particle stream. The resulting deposition upon the substrate forms a thin film.
Due to the high diffusion property of silver, low melting point of selenium, and the memory properties of silver-selenide, controlling the stoichiometry and morphology of the silver-selenide film during sputter deposition is difficult. For instance, silver-selenide bulk material is conductive, but its conductivity (about thousands ohm−1cm−1) is relatively lower than that of most metals. Also, silver concentration is critical for electrical performance of the device, thus it is necessary to maintain the silver concentration close to about 66.7 atomic weight percent (herein after represented “%”). With silver concentrations higher than about 67.5%, many nodular defects are formed in and/or on the silver-selenide film. The size of these defects can be about a tenth of a micrometer, which could have severe negative impact on sub micron device fabrication. Although the exact mechanism by which these defects are formed are unknown, it is believed that these defects are caused by excess silver, beyond the desired stoichiometric silver concentration requirements of the silver-selenide film.
It would be desirable to have an improved method of depositing a silver-selenide film. It would also be desirable to have a method of controlling the stoichiometry and morphology of silver-selenide for sputter deposition.